diff --git a/src/index.css b/src/index.css
index 9d287be..fdc338c 100644
--- a/src/index.css
+++ b/src/index.css
@@ -58,10 +58,15 @@ html.df-chat-open #root {
   background-color: #ffffff;
 }
 
-/* While any AppDialog is open, the reserved scrollbar-gutter strip must read
-   as fog, not app background. AppDialog toggles `df-dialog-open` on <html>
-   (ref-counted for stacked dialogs). The solid fog token is published by
-   src/colors.js (installCssVars) with a hard-coded fallback. */
+/* While any AppDialog is open, lock page scrolling in the background and ensure
+   the reserved scrollbar-gutter strip reads as fog, not app background. AppDialog
+   toggles `df-dialog-open` on <html> (ref-counted for stacked dialogs). The solid
+   fog token is published by src/colors.js (installCssVars) with a hard-coded fallback. */
+html.df-dialog-open,
+html.df-dialog-open body {
+  overflow: hidden !important;
+}
+
 html.df-dialog-open,
 html.df-dialog-open body,
 html.df-dialog-open #root {